178 OUTLINES OF CHORDATE DEVELOPMENT 



the mouth opens, the bulbus aortae becomes divided into the 

 anterior and posterior parts characteristic of the adult frog, 

 and in the former, now known as the truncus arteriosus, a 

 longitudinal fold appears separating its cavity into right and 

 left channels. 



2. The Origin of the Blood and Vessels 



Details regarding the exact method of origin of the blood 

 vessels of the frog are scanty. For the most part they seem to 



FIG. 63. Sections showing the formation of the blood islands in the frog. 

 After Brachet. A. Part of a transverse section through the middle of the yolk 

 region of a 2.8 mm. embryo of R. temporaries. B. Same of a 3.2 mm. embryo. 

 en, Endothelium; i, blood island; m, mesoderm. 



arise (4-4.5 mm.) as irregular and often isolated lacunar spaces 

 in the mesenchyme and splanchnic mesoderm. The cells bor- 

 dering these spaces gradually form a definite boundary and the 

 sinuses thus formed are linked into continuous vessels. In 



